diff: fix leaking orderfile option

The `orderfile` diff option is being assigned via `OPT_FILENAME()`,
which assigns an allocated string to the variable. We never free it
though, causing a memory leak.

Change the type of the string to `char *` and free it to plug the leak.
This also requires us to use `xstrdup()` to assign the global config to
it in case it is set.

This leak is being hit in t7621, but plugging it alone does not make the
test suite pass.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/combine-diff.c b/combine-diff.c
index 829a44e..f6b624d 100644
--- a/combine-diff.c
+++ b/combine-diff.c
@@ -1393,9 +1393,8 @@ static struct combine_diff_path *find_paths_generic(const struct object_id *oid,
 {
 	struct combine_diff_path *paths = NULL;
 	int i, num_parent = parents->nr;
-
 	int output_format = opt->output_format;
-	const char *orderfile = opt->orderfile;
+	char *orderfile = opt->orderfile;
 
 	opt->output_format = DIFF_FORMAT_NO_OUTPUT;
 	/* tell diff_tree to emit paths in sorted (=tree) order */